Use of selected oleophilic ethers in water-based drilling fluids of the O/W emulsion type and corresponding drilling fluids with improved ecological acceptability

ABSTRACT

The use is disclosed of at least substantially water-insoluble ethers which are fluid and/or at least plastically deformable at working temperature and have flash points of at least 80° C., of mono- and/or polyfunctional alcohols of natural and/or synthetic origin or corresponding solutions of such ethers in ecologically acceptable water-insoluble oils as the dispersed oil phase of water-based O/W-emulsion drilling fluids which are suitable for the environmentally acceptable development of geological formations and which contain, if desired, insoluble, finely particulate weighting agents for the formation of water-based O/W-emulsion drilling muds and/or further additives, such as emulsifiers, fluid-loss additives, wetting agents, alkali reserves and/or auxiliary substances for the inhibition of drilled rock of high water-sensitivity.

This application is a continuation of application Ser. No. 07/777,376filed on Nov. 20, 1991 now abandoned, which is the national stage ofPCT/EP90/00732, filed May 7, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention discloses new drilling fluids based on water-basedO/W-emulsions and O/W-emulsion drilling muds based thereon, which aredistinguished by high ecological acceptability with at the same timegood standing and application properties. An important area of use forthe new drilling mud systems is in off-shore wells for the developmentof petroleum and/or natural gas deposits, the aim of the invention beingparticularly to make available technically usable drilling fluids withhigh ecological acceptability. The use of the new drilling mud systemsadmittedly has particular significance in the marine environment, but isnot limited thereto. The new mud systems can also be put to quitegeneral use in land-based drilling, i.e. also serve for the developmentof petroleum and/or natural gas deposits here. They are, however, newvaluable working agents, for example, also in geothermal wells, in waterbore-holes, in the drilling of geoscientific bores and in drilling forthe mining industry. It is also essentially true here that theassociated ecotoxic problems are substantially simplified by the newwater-based O/W-drilling fluids selected according to the invention.

2. Discussion of Related Art

Liquid mud systems used in the sinking of rock bores for bringing up theloosened drill cuttings are known to be flowable systems, thickened to alimited extent, which can be assigned to one of the three followingclasses:

Purely aqueous drilling fluids; drilling mud systems based on oil, whichas a rule are used in the form of so-called invert emulsion muds, andrepresent preparations of the W/O-emulsion type in which the aqueousphase is distributed as a heterogeneous fine dispersion in thecontinuous oil phase. The third class of the known drilling fluids iscomposed of water-based O/W-emulsions, i.e. fluid systems which containa heterogeneous, finely-dispersed oil phase in a continuous aqueousphase. The invention discloses improved systems of this latter type.

The application properties of the drilling fluids of such O/W-emulsionsystems take an intermediate position between those of the purelyaqueous systems and those of the oil-based invert fluids. Theadvantages, but also the disadvantages, of the purely aqueous systemsare connected with the advantages and disadvantages of the oil-basedinvert-emulsions disclosed hitherto. Detailed information on thissubject can be found in the relevant specialist literature, refer, forexample, to the text book by George R. Gray and H. C. H. Darley,“Composition and Properties of Oil Well Drilling Fluids”, 4th. edition,1980/1981, Gulf Publishing Company, Houston, and the extensivespecialist and patent literature cited therein and to the manual“Applied Drilling Engineering”, Adam T. Bourgoyne, Jr. et al., FirstPrinting Society of Petroleum Engineers, Richardson, Tex. (USA).

One of the main weaknesses of purely water-based drilling mud systemlies in the interaction of water-sensitive, particularlywater-swellable, rock and/or salt formations with the aqueous drillingfluid and the secondary effects initiated thereby, in particularbore-hole instability and thickening of the drilling fluid. Manyproposals are concerned with the reduction of this problem area andhave, for example, resulted in the development of the so-calledinhibitive water-based muds, cf. for example, “Applied DrillingEngineering”, loc. cit., Chapter 2, Drilling Fluids, 2.4 and Gray andDarley loc. cit., Chapter 2, in particular the sub-section on pages 50to 62 (Muds for “Heaving Shale”, Muds for Deep Holes, Non-DispersedPolymer Muds, Inhibited Muds: Potassium Compounds).

In more recent practice, in particular drilling fluids based on oil,which consist of the 3-phase system oil, water and finely particulatesolids and are preparations of the W/O-emulsion type, have succeeded inovercoming the difficulties described above. Oil-based drilling fluidswere originally based on diesel oil fractions containing aromatics. Fordetoxification and to reduce the ecological problems created thereby, itwas then proposed to use hydrocarbon fractions which are largely free ofaromatics—now also known as “non-polluting oils”—as the continuous oilphase, see in this regard, for example, the publications by E. A. Boydet al. “New Base Oil Used in Low Toxicity Oil Muds”, Journal ofPetroleum Technology, 1985, 137-143 and R. B. Bennet “New Drilling FluidTechnology—Mineral Oil Mud”, Journal of Petroleum Technology, 1984,975-981 and the literature cited therein.

Drilling fluids of the water-based O/W-emulsion system type have alsohitherto used pure hydrocarbon oils as the dispersed oil phase, cf.here, for example, Gray, Darley loc. cit., p. 51/52 under the section“Oil Emulsion Muds” and the tabular summary on p. 25 (Tables 1-3) withdetails of water-based emulsion fluids of the salt-water mud, lime mud,gyp mud and CL-CLS mud type.

In this context in particular it is known that water-based O/W-emulsionfluids represent a substantial improvement in many regards to the purelywater-based drilling mud systems. Particularly in more recent times,however, the advantages and disadvantages of such water-based emulsionfluids have also been examined critically in comparison with theoil-based invert-systems. This is due to the considerable ecologicalreservations felt towards the oil-based invert drilling fluids commonlyused today.

These ecological reservations can be subdivided into two problem areas:

In addition to the basic constituents, i.e. oil and water, all drillingfluid systems based on water and/or oil require a large number ofadditives for the establishment of the desired application properties.The following can be mentioned here purely by way of example:emulsifiers or emulsifier systems, weighting agents, fluid-lossadditives, wetting agents, alkali reserves, viscosity regulators, insome cases auxiliary agents for the inhibition of drilled rock with highwater-sensitivity, biocides and the like. A detailed summary can befound, for example, in Gray and Darley, loc. cit., Chapter 11, “DrillingFluid Components”. The industry has developed additives which currentlyappear ecologically harmless, but also additives which are ecologicallyquestionable or even ecologically undesirable.

The second problem area is caused by the oil phases used in suchdrilling fluids. Even the hydrocarbon fractions which are largely freefrom aromatics, currently known as “non-polluting oils”, are notcompletely harmless when released into the environment. A furtherreduction in the environmental problems, which are caused by the fluidoil phases of the type referred to here, appears urgently necessary.This is true in particular for the sinking of off-shore wells, e.g., forthe development of petroleum or natural gas deposits, because the marineeco-system reacts particularly sensitively to the introduction of toxicand poorly degradable substances.

There have recently been same proposals for reducing these latterproblems. For example, the U.S. Pat. Nos. 4,374,737 and 4,481,121disclose oil-based invert-drilling fluids in which non-polluting oilsare to be used. The following can be mentioned together as of equalvalue as non-polluting oils: mineral oil fractions which are free fromaromatics, and vegetable oils, such as peanut oil, soybean oil, linseedoil, corn oil, rice oil or even oils of animal origin, such as whaleoil. These named ester oils of vegetable and animal origin are all,without exception, triglycerides of natural fatty acids, which are knownto have high environmental acceptability and are clearly superior fromthe ecological point of view to hydrocarbon fractions—even when these donot contain aromatic hydrocarbons.

In the above U.S. patent No., however, not one concrete exampledescribes the use of such natural ester oils in invert drilling fluids.Without exception, mineral oil fractions are used as the continuousoil-phase. In fact, oils of vegetable and/or animal origin of the typementioned here are not considered for invert drilling fluids forpractical reasons. The rheological properties of such oil phases cannotbe controlled over the wide temperature range generally required inpractice, from 0 to 5° C. on the one hand, up to 250° C. and more on theother.

The Applicant's earlier proposals

A series of earlier Applications by the Applicant describes the use ofeasily biodegradable and ecologically harmless ester oils as thecontinuous oil phase in W/O-invert drilling mud systems. Refer here inparticular to the earlier Applications P 38 42 659.5 and P 38 42 703.6(U.S. Ser. No. 07/452,457 filed Dec. 18, 1989 now abandoned and U.S.Ser. No. 07/452,988 filed Dec. 19, 1989 now abandoned) and themodifications of the ester oils that can be used according to thedetails of the earlier Patent Applications P 39 07 391.2 and P 39 07392.0.

The subject of these earlier Applications is the use of ester oils basedon selected monocarboxylic acids and monocarboxylic acid mixtures andmonofunctional and optionally polyfunctional alcohols as the continuousoil phase in W/O-invert systems. The earlier Applications show that,using the esters and ester mixtures disclosed therein, not only cansatisfactory rheological properties be established in the fresh drillingfluid, but it is also possible by the additional use of selected, knownalkali reserves in the drilling fluid, for work to be carried outwithout fearing undesired thickening effects when there is a partialester hydrolysis.

An important further development of such invert drilling fluids based onester oils is the subject of the Applicant's earlier Application P 39 03785.1 (U.S. Ser. No. 07/478,185 filed Feb. 9, 1990 now abandoned).

The teaching of this earlier Application starts with the concept of alsousing a further additive in invert drilling fluids based on ester oilswhich is suitable for keeping the desired rheological data of thedrilling fluid in the required range, even when in practice larger andlarger amounts of free carboxylic acids are formed by partial esterhydrolysis. The Application provides for the additional use of basicamine compounds which are capable of forming salts with carboxylic acidsand have a marked oleophilic nature and at most limitedwater-solubility, as additives in the oil phase.

A modified form of such W/O-invert drilling mid systems is disclosed inthe Applicant's earlier Application P 39 11 238.1 which discloses theuse of:

a) at least largely water-insoluble mono- and/or polyfunctional alcoholsof natural and/or synthetic origin which are fluid and pumpable in thetemperature range of 0 to 5° C.

or of

b) solutions, which are fluid and pumpable in the given temperaturerange, of at least largely water-insoluble mono- and/or polyfunctionalalcohols of natural and/or synthetic origin in ecologically acceptablewater-insoluble oils as the continuous oil phase of drilling fluidswhich exist in W/O-emulsion form and in the alcohol-containing oil phasehave a dispersed aqueous phase and also, if desired, furtherconventional additives. Ester oils are provided, at least in part, asthe ecologically acceptable, water-insoluble oils for admixture with thewater-insoluble alcohols, such as described in the aforementionedearlier Applications of the Applicant.

The Applicant's earlier Application P 39 11 299.3, finally, disclosesthe use of water-insoluble ethers having flash points above 80° C. ofmonohydric alcohols of natural and/or synthetic origin with at least 4carbon atoms, preferably at least 6 carbon atoms, in the alcoholradicals as the oil phase or a constituent of the oil phase of suchinvert drilling fluids as exist in W/O-emulsion form. The additional useof the aforementioned ester oils for admixture with the ethers is alsoprovided here.

DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients or reaction conditions usedherein are to be understood as modified in all instances by the term“about”.

The invention problem and its technical solution

The present invention starts with the problem of providing drilling mudsystems of the highest, and in this form previously unknown, ecologicalacceptability which simultaneously have good application properties andwhich also in particular enable satisfactory application even in problemareas. In the invention a conscious decision has therefore been made toreject the oil-based type of invert drilling mud systems and to returnto the oil-modified water-based type of O/W-emulsion systems. At thesame time, however, the auxiliary agents described in the cited earlierApplications of the Applicant, and the ecological advantages associatedtherewith, are now also to be used in this class of drilling mudsystems.

In a first embodiment, the intention of the invention is therefore tomake use of the advantages that O/W-emulsion mud systems have againstpurely water-based drilling fluids, but at the same time to replace themineral-oil phase at least to a substantial amount—completely orpartially—with ecologically harmless ethers of marked oleophiliccharacter.

In a further approach, the invention aims also to reduce the ecologicalconcerns of the second problem area, i.e. that associated with theadditives and auxiliary agents in drilling fluids, by selecting from thewide range of additives known in this field, at least to a large extentand preferably in all cases, those auxiliary agents which aredistinguished by their ecologically harmless nature.

The subject of the invention is accordingly in a first embodiment theuse of at least substantially water-insoluble ethers, which are fluidand/or at least plastically deformable at working temperature and haveflash points of at least 80° C., of mono- and/or polyfunctional alcoholsof natural and/or synthetic origin or corresponding solutions of suchethers in ecologically acceptable water-insoluble oils as the dispersedoil phase of water-based O/W-emulsion drilling fluids, which aresuitable for an environmentally friendly development of geologicalformations, and, if desired, contain insoluble, finely particulateweighting agents for the formation of water-based O/W-emulsion drillingmuds and/or further additives, such as emulsifiers, fluid-lossadditives, wetting agents, alkali reserves and/or auxiliary substancesfor the inhibition of drilled rock with high water-sensitivity.

In a further embodiment, the invention relates to water-basedO/W-emulsion drilling fluids, which in a homogeneous aqueous phasecontain, in stable dispersion, an oil phase in amounts of about 5 to 50%by weight—the percentage by weight referred to the sum of the unweightedwater phase and oil phase—together, if required, with dissolved and/ordispersed auxiliary substances such as emulsifiers, fluid-lossadditives, wetting agents, finely particulate weighting agents, salts,alkali reserves and/or biocides and are characterized in that at least asubstantial part of the dispersed oil phase is formed bywater-dispersible ethers which are fluid or at least plasticallydeformable at working temperature, and have flash points of at least 80°C., from mono- and/or polyfunctional alcohols of natural and syntheticorigin or corresponding solutions of such ethers in ecologicallyacceptable, water-insoluble oils, in particular oleophilic, mono- and/orpolyhydric alcohols and/or corresponding ester oils.

Both embodiments of the teaching according to the invention include theadditional preferred step of also using for the water-based emulsiondrilling fluids or emulsion drilling muds, at least to a large extent,those inorganic and/or organic auxiliary and loading substances whichare at least predominantly ecologically and toxicologically harmless.Thus, for example, in the most important embodiments of the inventionthe use of auxiliary agents based on soluble, toxic heavy-metalcompounds is avoided.

The preferred embodiments of the invention

The mixture ratios of the ether-oil/water phases cover the usual rangefor the previously known O/W-emulsion drilling fluids based on mineraloils. The lower limit values for the oil phase are usually at leastabout 5% by weight, or preferably between about 5 and 10% by weight,e.g., therefore 7 or 8% by weight—each percentage by weight referred tothe total weight of the fluid phases oil+water, each in the unweightedstate. Minimum amounts of the order given ensure that use can be made ofthe characteristic peculiarities of an O/W-emulsion fluid. The upperlimit value for the oil content is usually about 50% by weight or evenslightly higher, e.g., a maximum of about 65% by weight. Assuming thatthe droplet size of the dispersed oil phase is sufficiently even, therange with the most dense packing is therefore already achieved, andthus conversion into the fluid type of the W/O-invert fluids is obviousor appears logical.

The upper limit for the content of dispersed oil phase in the O/W-fluidsaccording to the invention is generally determined by cost/benefitconsiderations and is, for example, about 45% by weight, preferablyless, e.g., about 40% by weight.

An ether content of about 10 to 40% by weight—percentage by weightcalculated as before—and in particular amounts of the ether phase ofabout 15 to 35% by weight give the possibility of exploiting many—knownand not previously described—advantages of such emulsion fluids. Oilcontents of, for example, 20% by weight, or in an extreme case, 30% byweight provide the basis for high-quality drilling fluids which at leastcome very close in the way they function to the oil-based invert fluids,but require very much less ether phase.

The various embodiments of the invention

In a first embodiment the dispersed oil phase of the water-basedO/W-drilling fluids is formed exclusively, or to much the largest part,by the essentially water-insoluble and preferably markedly oleophilicethers. The rheology of the ethers used is adapted here to the technicalrequirements of the drilling fluids, slight rheological corrections arepossible by adding the small quantities of thinners provided in thisembodiment. In the case described here, consideration can be given inparticular to dispersed oil phases which are formed to more than 70% byweight, preferably to more than 80% by weight and, if required,exclusively by the ethers themselves.

The oil-mixture components optionally added in small amounts in thisembodiment can be pure hydrocarbon compounds, particularly those freefrom aromatics, but in particular selected ester oils and/or oleophilicalcohols of the type described in the above earlier Applications of theApplicant. This embodiment will be discussed in more detail below.

A second embodiment of the invention relates to the use of dispersedoil-phases in systems of the type referred to here, which haveconsiderable or even predominant amounts of non-water-miscible oils,which are not identical with the oleophilic ethers, but are present inadmixture with them.

In this embodiment, the content of the ethers selected according to theinvention in the dispersed oil phase is in the region of at least about10% by weight to about 70% by weight—each referred to the fluid oilphase—in which ether proportions can be preferred in quantities of atleast about 35% by weight, and preferably at least about 50% by weight,of the oil phase. Ecologically harmless ester oils of the type to bedescribed in detail below are preferred mixture components of thissecond embodiment also. The additional use of pure hydrocarbon oils isnot, however, excluded.

The ethers of marked oleophilic character used according to theinvention

The use of ethers as the oil phase, but also their combined use insmaller or larger amounts in the oil phase, requires that these ethersare sufficiently insoluble in water. The water-solubility of suitableethers at room temperature is preferably less than 1% by weight andpreferably not more than about 0.5% by weight.

The following general rules apply for the chemical nature of the ethers:Ethers of monofunctional and/or polyfunctional alcohols are suitable. Inparticular, besides the ethers of monofunctional alcohols, those ofdifunctional and polyfunctional alcohols can be considered. The alcoholsthemselves should be ecologically acceptable and thus should have noaromatic constituents in the preferred embodiment. The preferredether-forming compounds are straight-chain and/or branched aliphatic, oralso corresponding unsaturated, in particular mono- and/orpoly-olefin-unsaturated alcohols. Cycloaliphatic alcohols or ether canbe considered.

An important general requirement in the sense of the treatment accordingto the invention is that these alcohols or ethers as such are not onlyecologically acceptable, but also do not cause any other toxicologicalhazards, particularly when inhaled.

From the wide range of suitable ethers, preferably based at least partlyon markedly oleophilic alcohols, in particular those ethers can beconsidered of alcohols with a straight-chain and/or branched hydrocarbonstructure. From the group of corresponding monofunctional alcohols,corresponding compounds with at least 5, preferably at least 6 or 7carbon atoms are suitable compounds, with corresponding alcohols with atleast 8 carbon atoms in the molecule being particularly usable. Theupper limit for the number of carbon atoms is determined by technicalaccessibility and is, for example, in the region of about 36, preferablyabout 20 to 24. The ether-forming alcohols themselves can bestraight-chain and/or branched, they can be aliphatic-saturated or alsomono- and/or poly-olefin-unsaturated.

Essentially, ethers from alcohols of natural and/or synthetic origin aresuitable. In particular, the synthetic alcohols of the range of about 8to 24 carbon atoms, which is of particular interest here, which can alsocontain unsaturated parts, are frequently low-cost components which areavailable commercially and can be used for the purposes of theinvention.

The ethers themselves should have flash points of at least 80° C.,preferably of at least 100° C. and in particular of at least 120° C. Aswell as the named ethers based on monofunctional alcohols withpreferably at least 8 carbon atoms, ethers of selected polyols can alsobe considered here. Suitable polyols for the formation of the ethers arein particular lower diols, such as ethylene glycol and/or the propyleneglycols, but also in some cases branched-chain diols with a higheroleophilic hydrocarbon radical in the molecule. For example, oleophilicdiols are suitable with hydroxyl groups in the alpha, omega positionand/or longer chain diols, which have their hydroxyl groups on adjacentcarbon atoms. Typical examples of compounds of this type are2,2-dimethyl-1,3-propanediol (neopentyl glycol) or the saponificationproducts of epoxidized olefins.

The mixture components in the dispersed oil-phase

Suitable oil components for the admixture according to the invention arethe mineral oils currently used in practice in drilling fluids, andpreferably the aliphatic and/or cycloaliphatic hydrocarbon fractionswhich are essentially free from aromatics. Refer here to the relevantprior-art publications and the commercial products available on themarket.

Particularly important mixture components are, however, oleophilicalcohols and/or ester oils which are ecologically acceptable in thesense of the invention, and to which the following generalconsiderations here immediately apply:

The alcohols or ester oils should be fluid at ambient temperature, butalso under the conditions of use, with the range of materials thought tobe fluid, however, also including materials which are at leastplastically deformable at ambient temperature and which soften to becomefluid at the usually elevated working temperatures. For reasons ofeasier processability in practice, alcohols and ester oils withsolidification values (pour point and setting point) below 10° C. andusefully below 0° C. are preferred. Corresponding compounds withsolidification values not above −5° C. can be particularly suitable. Oneshould take into account here the fact that the drilling fluids areusually produced on site using, for example, sea water at comparativelylow water-temperatures.

For reasons of operational safety, it must be a requirement that theadditives have flash points of at least 80° C., however, higher flashpoints of at least 100° C. and substantially higher values arepreferred, for example, those above 150 or 160° C.

A further important requirement for the optimal employment of thesubject of the invention is that these alcohols and/or ester oils shouldhave a biologically or ecologically acceptable constitution, i.e. inparticular that they are free from undesired toxic constituents. In thepreferred embodiment of the invention, alcohols and/or ester oils areaccordingly used which are free from aromatic constituents and inparticular have saturated and/or olefin-unsaturated, straight-chainand/or branched hydrocarbon chains. The use of components containingcycloaliphatic structural constituents is possible from the ecologicalpoint of view, but for reasons of cost will be of little significance inpractice.

Carboxylic acid esters of the type in question here are subject to alimited degree, as the highly dispersed oil-phase in a continuousaqueous phase, to hydrolytic ester-cleavage with liberation of theester-forming constituents, i.e. carboxylic acid and alcohol. Twoclosely linked issues should be taken into account as regards theapplication properties of the ester oils in the sense of the invention,namely considerations of the possible toxicity of the liberatedcomponents, in particular of the alcohol components, when inhaled andalso any change in the composition of the emulsion fluid and associatedchange which may take place in its application properties.

For comprehension of the teaching according to the invention, theseconsiderations should be examined separately for each of theester-forming basic constituents—on the one hand the alcohols and on theother the carboxylic acids.

According to the invention, both monohydric alcohols and polyhydricalcohols are suitable as the ester-forming alcohol components, and anymixtures of these types can also be used. A further distinction can bemade between alcohols according to their solubility behavior in water.Alcohols can be water-soluble and/or water-insoluble.

In a first group, polyhydric alcohols are to be considered. Particularlypreferred here are the industrially easily available lower,polyfunctional alcohols with 2 to 5, preferably 2 to 4 hydroxyl groupsand in particular 2 to 6 carbon atoms, which form esters of suitablerheology.

Typical representatives are ethylene glycol, the propanediols andparticularly glycerin.

Polyhydric alcohols of the type referred to here are distinguished byhigh water-solubility and at the same tire by such low volatilizationvalues that considerations of the exclusion of toxic hazards wheninhaled do not usually apply.

Polyhydric lower alcohols of the type referred to here can be used astotally esterified oil components and/or as partial esters with am freehydroxyl groups and/or can be formed in the practical use of theemulsion fluid according to the invention. Provided that the partialesters formed retain the at least largely water-insoluble character ofthe oil phase, no substantial change takes place in the oil/water ratioin the emulsion fluid. The situation only becomes different whenwater-soluble hydrolysis products form—in particular therefore the freelower polyhydric alcohols. The changes which occur in practicaloperation in such emulsion fluids as a result of this process are,however, insignificant. Firstly, a comparatively high stability of theester bond is ensured under the working conditions according to theinvention. O/W-emulsion fluids are known usually to operate in thepH-range of approx. neutral to moderately alkaline, for example, in thepH-range of about 7.2 to 11 and in particular about 7.5 to 10.5, andthus for these reasons alone there is no aggressive hydrolytic attack onthe ester bond. In addition, and moreover, the following is inparticular also true:

In the practical use of the drilling fluid, and the associated drivingforward of the bore into ever deeper earth strata, a continualconsumption of the drilling fluid and in particular of the oil-phaseused in the drilling fluid takes place. Emulsion fluids are known—andthis is an important advantage of their use—for the fact that theemulsified oil phase clings on to solid surfaces and therefore bothseals the filter bed to the wall of the bore-shaft and hinders, or evenprevents, interaction between the drilled rock and the aqueous phase ofthe drilling fluid. This continual consumption of drilling fluid, and inparticular of the oil-phase, necessitates a continual supply ofoil-based mud. In practical operation, a state of equilibrium istherefore rapidly established in the drilling fluid which governs andmakes possible continuous operation for long periods of time.

The above considerations, naturally, are only important in the contextof the present invention when not inconsiderable amounts of thedispersed oil phase are formed by the ester oils also used. Theoleophilic ethers and their admixture with hydrolysis-resistant mixtureconstituents are not affected by such additional considerations and sothat largely, or even completely, dispensing with ester oils can beregarded as an important and preferred embodiment of the use of theinvention.

From some viewpoints, further considerations should be taken intoaccount when monohydric alcohols are used as the ester-formingconstituent of the ester oils. Here only the lower members of thesealcohols are water-soluble or, in an unlimited quantity, water-miscible.However, their volatility can also be regarded as an importantconsideration in the case of these alcohols. In the practical operationof drilling work, at least moderately increased temperatures are rapidlyestablished in the circulated drilling fluid and therefore the partsexposed by the pumping to remove the drill cuttings have a temperature,for example, in the region of 50 to 70° C. Considerations oftoxicological effects when inhaled must therefore be taken into accounthere. Even C₄-alcohols, e.g., isobutyl alcohol, can be so volatile underthe operating conditions on the drilling platform that possible hazardsto personnel must be taken into account. According to the invention,therefore, when ester oils are employed together with monohydricalcohols, the lower carbon number limit for these monohydric alcohols ispreferably selected as 6, and working with esters of monofunctionalalcohols with at least 8 carbon atoms can be particularly preferred.

The selection and limitation of the carbon number in the ester-formingalcohol, however, at the same time has the following result as regardsthe composition of the ester-oil phase when there is a partialhydrolysis during operation: The hydrolyzing parts of such ester oilsare converted to the free alcohol which remains as a practicallywater-insoluble mixture constituent in the dispersed ester-oil phase.

A number of points must also be considered with regard to the carboxylicacids formed by the partial hydrolysis of the related ester oils.

Here it is possible, depending on the specific constitution of thecarboxylic acids used, to distinguish two basic types—with a smoothtransition between the two: carboxylic acids which give rise tocarboxylic acid salts with an emulsifier effect, and those which giverise to inert salts.

The respective chain-length of the liberated carboxylic acid molecule isparticularly decisive here. Further, the salt-forming cation usuallypresent in the alkali reserves of the drilling fluid should also betaken into consideration.

In general the following rules apply here: Lower carboxylic acids, forexample those with 1 to 5 carbon atoms, give rise to the formation ofinert salts, for example, the formation of corresponding acetates orpropionates. Fatty acids of higher chain-length and in particular thosewith from 12 to 24 carbon atoms result in compounds with an emulsifiereffect.

It is therefore possible, by selecting suitable ester oils—and to acertain extent also the salt-forming cations in the emulsion fluid—tocontrol specifically the secondary products in the emulsion fluid, whichcan have considerable influence on the nature and effect of the emulsionfluid. What has been said above, again applies here: It is not only thedispersed organic phase, but also the aqueous phase, which is subject tocontinual consumption in practice and thus requires replacement. Instationary operation, therefore, controllable states of equilibrium willrapidly be established, even as regards the reaction by-products basedon the ester-forming carboxylic acids as discussed here.

General details for the definition of ester oils suitable as mixturecomponents

In the sense of the invention, the corresponding reaction products ofmonocarboxylic acids with monofunctional and/or polyfunctional alcoholsof the type indicated are preferred as the ester oils. The additionaluse of polyvalent carboxylic acids is not, however, excluded, but theyare of less significance, in particular for reasons of cost.

The carboxylic acids here can be of natural and/or synthetic origin,they are, as already indicated, preferably straight-chain and/orbranched and optionally cyclic, but not aromatic in structure. Theester-forming carboxylic acids can be saturated and/or unsaturated, withunsaturated compounds here being understood, in particular, to beolefin-unsaturated compounds, which can be either mono- orpoly-olefin-unsaturated. Olefin-unsaturated components can be ofparticular importance for setting the predetermined rheology values. Itis known that olefinic longer-chain compounds are suitable ascorresponding saturated components for the formation of esters withlower melting points.

The preferred range for the carbon number of the carboxylic acidsextends from 1 to 36 and in particular from 2 to 36. For reasons of easyavailability, an upper limit for the carbon number can be about 22 to24. The respective chain length of the ester-forming carboxylic acidcomponents is selected—depending on the nature of the alcoholscomponent(s) used—by consideration of the numerous issues alreadydiscussed, and refers not only, for example, to the ester and/or itsrheology directly, but also to the reaction by-products formed inparticular by partial hydrolysis.

Suitable alcohols are, as indicated, both monofunctionalalcohols—provided the above limitations are taken into consideration—andpolyfunctional alcohols, particularly lower polyfunctional alcohols with2 to 6 carbon atoms and preferably with a maximum of 4 hydroxyl groups.

The alcohol components here can also be of natural and/or syntheticorigin, they are straight-chain or branched and in particular in thecase of the monofunctional alcohols are saturated and/or alsoolefin-unsaturated. Monofunctional alcohols have in particular up to 36carbon atoms, preferably up to about 24 carbon atoms. Alcohols with 6 to18, in particular 7 to 15 carbon atoms, of natural and/or syntheticorigin can only be of particular significance in the formation of theester oils.

Particularly important ester oils in the sense of the use in theinvention are the ecologically-acceptable ester oils as described inparticular in the cited earlier Applications P 38 42 659.5, P 38 42703.6, P 39 07 391.2 and P 39 07 392.0 (U.S. Ser. No. 07/452,457 filedDec. 18, 1989 now abandoned, U.S. Ser. No. 07/452,988 filed Dec. 19,1989 now abandoned and. To complete the invention disclosure, theessential characteristics of these ester oils or ester mixtures arebriefly summarized below.

The dispersed ester-oil phase accordingly contains carboxylic acidesters from at least one of the following sub-classes:

a) Esters from C₁₋₅-monocarboxylic acids and mono- and/or polyfunctionalalcohols, in which radicals from monohydric alcohols have at least 6,preferably at least 8 carbon atoms, and the polyhydric alcoholspreferably have 2 to 6 carbon atoms in the molecule,

b) esters from monocarboxylic acids of synthetic and/or natural originwith 6 to 16 carbon atoms, in particular esters of correspondingaliphatic-saturated monocarboxylic acids and mono- and/or polyfunctionalalcohols of the type indicated under a),

c) esters of olefin mono- and/or poly-unsaturated monocarboxylic acidswith at least 16, in particular 16 to 24 carbons atoms, and inparticular monofunctional straight-chain and/or branched alcohols.

The latter esters of olefin mono- and/or poly-unsaturated monocarboxylicacids with at least 16 carbon atoms (c) are preferably assigned to atleast one of the following sub-classes:

c1) esters which are derived by more than 45% by weight, preferably bymore than 55% by weight from di- and/or poly-olefin-unsaturatedC₁₆₋₂₄-monocarboxylic acids,

c2) esters which are derived by not more than 35% by weight from di- andpoly-olefin-unsaturated acids, and are preferably at least about 60% byweight mono-olefin-unsaturated.

The raw materials for obtaining many of the monocarboxylic acids inthese sub-classes, in particular those with a higher carbon number, arevegetable and/or animal oils. Coconut oil, palm kernel oil and/orbabassu oil, can be mentioned in particular as materials used forobtaining monocarboxylic acids mainly in the range with up to 18 carbonatoms, and with essentially saturated components. Examples of vegetableester oils, in particular for olefin mono- and optionallypoly-unsaturated carboxylic acids with from 16 to 24 carbon atoms, arepalm oil, peanut oil, castor oil and in particular rapeseed oil.Carboxylic acids of animal origin of this type are in particularcorresponding mixtures of fish oils, such as herring oil.

The teaching of the invention expressly includes also and in particularthe use of monocarboxylic-acid triglycerides and therefore in particularalso the use of corresponding glyceride oils of natural origin. Here,however, the following must be considered: Natural oils and fats usuallyoccur in a form so highly contaminated, for example, with freecarboxylic acids or other accompanying substances, that there is as arule no question of immediately processing them in O/W-emulsion fluidsof the type referred to here. If such natural feed materials are addedin the commercially available form to water-based drilling fluids, thenalmost immediately, such a large amount of foam forms in the drillingfluid being used as to constitute a serious hindrance or even to resultin the drilling fluid becoming unusable. This may not be the case ifcleaned and/or synthetically produced selected triglycerides are used inthe dispersed oil phase. The teaching according to the invention can berealized without exception with these. In principle, however, with suchesters of high-grade alcohols one must always anticipate a notinconsiderable tendency to foam formation. Partial esters ofglycerin—the mono- or di-glycerides—are known to be effective emulsifiercomponents.

As already indicated, it is not only the comparatively low-viscosityester oils as in the disclosure of the cited earlier Applications of theApplicant in the field of invert drilling fluids based on ester-oils,which are suitable for the purposes of the invention, but within theframework of O/W-emulsion fluids, in particular comparatively viscousester oils can be of advantage as components of the dispersed phase.They are, for example, valuable auxiliary agents for sealing the finestpores in the filter cake of the bore shaft, or in rendering inertswellable rock. The lubricating ability of such comparativelyhigh-viscosity ester oils even at elevated temperatures in thebore-shaft, in particular also in deviated bore-holes, is in some casesdistinctly better than that of the comparatively thin-bodied ester oils.A dispersed ester oil phase of comparatively higher-viscosity ester oilsdoes not cause any detrimental effect on the drill-technology, therheology of the system as a whole is determined by the continuousaqueous phase. In this sense it may be preferred to use ester oils asthe dispersed phase which have a Brookfield viscosity of up to about500,000 mPa.s, preferably up to about 1 million mPa.s or even higher,for example, up to about 2 million mPa.s (determined at ambienttemperature). This constitutes an important extension of the teaching inthe named earlier Applications of the Applicant in the field ofoil-based invert drilling fluids based on ester-oils.

In one embodiment of the invention, branched-chain components and inparticular alpha-branched-chain alcohols and/or carboxylic acids can beof particular significance. Branches of this type are known on the onehand to influence rheology, the esters formed by such chain-branchingare usually more mobile. Furthermore, such alpha-branching can,moreover, also promote increased hydrolysis stability under workingconditions, which is therefore exploited in the invention.

The oleophilic alcohols suitable as possible mixture components

In particular, oleophilic alcohols as disclosed in the Applicant'searlier Patent Application P 39 11 238.1 (D 8511), as cited above, areconsidered. The most important embodiment thereof has as its subjectoleophilic monofunctional alcohols as described in the context of thepresent invention description for the formation of the ester oils.Reference should be made to these details.

The aqueous phase

All types of water are suitable for the production of the O/W-emulsionfluids according to the invention. These can therefore be based on freshwater and in particular also on salt water—particularly sea water foruse in off-shore wells.

Additives in the emulsion fluid

In principle, all the additives provided for parable drilling fluidtypes are considered here, which are usually added in connection with aquite specific desired range of drilling fluid properties. The additivescan be water-soluble, oil-soluble and/or water- or oil-dispersible.

Classical additives for water-based O/W-emulsion fluids can be:emulsifiers, fluid-loss additives, structure-viscosity-building solubleand/or insoluble substances, alkali reserves, agents for the inhibitionof undesired water-exchange between drilled formations—e.g.water-swellable clays and/or salt strata—and the water-based drillingfluid, wetting agents for better adhesion of the emulsified oil phase tosolid surfaces, e.g. for improving the lubricating effect, but also forthe improvement of the oleophilic seal of exposed rock formations, orrock surfaces, biocides, e.g., for inhibiting bacterial attack on suchO/W-emulsions, and the like. For details, reference should be made hereto the relevant prior art, as described in detail in the specialistliterature cited above, refer here in particular to Gray and Darley,loc. cit., Chapter 11, “Drilling Fluid Components”. We will thereforeonly cite extracts below:

Finely-dispersed additives for increasing the fluid density: Bariumsulfate (barite) is widely used, but also calcium carbonate (calcite) orthe mixed carbonate of calcium and magnesium (dolomite).

Agents for building up the structure-viscosity, which at the same timealso act as fluid-loss additives: First of all bentonite should bementioned here which is known to be used in water-based fluids in anon-modified form and is therefore ecologically safe. For salt-waterfluids other comparable clays, in particular attapulgite and sepiolite,are of considerable significance in practice.

The additional use of organic polymer compounds of natural and/orsynthetic origin can also be of considerable importance in this context.The following should in particular be mentioned here: starch orchemically modified starches, cellulose derivatives, such ascarboxymethylcellulose, guar gum, xanthan gum, or also purely syntheticwater-soluble and/or water-dispersible polymer compounds, such as inparticular the polyacrylamide compounds of high molecular weight with orwithout anionic or cationic modification.

Thinners for viscosity-regulation: So-called thinners can be of organicor inorganic nature; exiles of organic thinners are tannins and/orquebracho extract. Further examples are lignite and lignite derivatives,particularly lignosulfonates. As indicated above, in a preferredembodiment of the invention the use of toxic components is particularlyto be excluded, and in particular the corresponding salts with toxicheavy metals, such as chromium and/or copper. Polyphosphate compoundsare examples of inorganic thinners.

Emulsifiers: For the teaching according to the invention, two featuresin particular should be considered. It has emerged that a stabledispersion of ethers, alcohols and in some cases ester oils can be verymuch more easily possible than the corresponding dispersion of puremineral oils as used in the state of the art. This in itself is a firstsimplification. Furthermore, it should be considered that, whenlonger-chain carboxylic acid esters are used, by the partialsaponification of the ester oils under the additional effect of suitablealkali reserves, effective O/W-emulsifiers are also formed whichcontribute to the stabilization of the system.

Additives which inhibit undesired water-exchange with, for example,clays: The additives known from the state of the art for use inwater-based drilling fluids are considered here. In particular, this isconcerned with halides and/or carbonates of the alkali and/oralkaline-earth metals, with particular importance given to correspondingpotassium salts, optionally in combination with lime. Reference is madefor example to the appropriate publications in “Petroleum EngineerInternational”, September 1987, 32-40 and “World Oil”, November 1983,93-97.

Alkali reserves: inorganic and/or organic bases adapted to the generalbehavior of the fluid can be considered, in particular correspondingbasic salts or hydroxides of alkali and/or alkaline-earth metals andorganic bases.

In the field of organic bases, a conceptual distinction must be drawnbetween water-soluble organic bases—for example, compounds of thediethanolamine type—and practically water-insoluble bases of markedoleophilic character as described in the Applicant's earlier ApplicationP 39 03 785.1 (U.S. Ser. No. 07/458,195 filed Feb. 9, 1990 nowabandoned), cited above, as additives in invert drilling muds based onester oil. The use of such oil-soluble bases in the framework of thepresent invention in particular falls within the new teaching.Oleophilic bases of this type, which are distinguished in particular byat least one longer hydrocarbon radical with, for example, 8 to 36carbon atoms, are, however, not dissolved in the aqueous phase but inthe dispersed oil phase. Here these basic components are of multiplesignificance. On the one hand they can act directly as alkali reserves.On the other, they give the dispersed oil droplets a certain positivecharge and therefore result in increased interaction with negativesurface charges which can be found in particular in the hydrophilicclays which are susceptible to ion-exchange. According to the inventionthe hydrolytic cleavage and the oleophilic sealing of water-reactiverock strata can thus be influenced.

The quantity of auxiliary substances and additives used in each casemoves essentially within the usual boundaries and can therefore be takenfrom the cited relevant literature.

EXAMPLES

Firstly, a 6% by weight homogenized bentonite suspension is preparedusing commercially available bentonite (not hydrophobized) and tapwater, and a pH value of 9.2 to 9.3 is adjusted by means of a sodiumhydroxide solution.

Starting with this pre-swollen aqueous bentonite phase, the individualcomponents of the water-based emulsion as in the following formulationare incorporated in successive stages of the process—each underintensive intermixing:

350 g 6% by weight bentonite solution

1.5 g industrial carboxymethylcellulose low-viscosity (Relatin U 300 S9)

35 g sodium chloride

70 g ether (as in the definition given below)

1.7 g emulsifier (sulf. castor oil “Turkey-red oil”)

219 g barite

Viscosity measurements are carried out on the thus prepared O/W-emulsionfluids as follows:

Firstly, the plastic viscosity (PV), the yield point (YP) and the gelstrength after 10 sec. and after 10 min. of the emulsion fluid aredetermined at 50° C. on the unaged material.

The emulsion fluid is then aged for 16 hours at 125° C. in an autoclavein the so-called “roller-oven” to examine the effect of temperature onthe stability of the emulsion. Then the viscosity values at 50° C. aredetermined once again. In each of the following examples the nature ofthe ether used, the values determined for the unaged and aged materialand—if necessary—general comments are given.

Example 1

unaged aged material material plastic viscosity [mPa.s] 10 13 yieldpoint [Pa] 15.3 9.6 gel strength [Pa] 10 sec. 10.5 6.7 10 min. 11.5 9.1

Example 2

The formulation in Example 1 is repeated, but without using theemulsifier (Turkey-red oil).

The viscosity values measured on the unaged and aged material are asfollows:

unaged aged material material plastic viscosity [mPa.s] 8 7 yield point[Pa] 16.2 17.7 gel strength [Pa] 10 sec. 17.2 18.2 10 min. 16.8 13.4

Even in the fresh formulation a slight droplet formation can be seen onthe surface.

Example 3

Oil phase used: di-isotridecylether

The viscosity values measured on the unaged and the aged drilling fluidare as follows:

unaged aged material material plastic viscosity [mPa.s] 15 15 yieldpoint [Pa] 11.5 14.4 gel strength [Pa] 10 sec. 11.0 13.4 10 min. 16.812.5

Examples 4 and 5 (Comparative Examples)

For comparison with the emulsion fluids based on ethers according to theinvention, emulsion fluids are made up according to the formulationgiven above—once with and once without the emulsifier Turkey-redoil—based on a pure hydrocarbon oil and tested. The commercial productU.S. Ser. No. 07/458,185 filed Feb. 9, 1990 now abandoned, which isemployed widely in drilling fluids in current practice, is used as thehydrocarbon oil.

The table of values for Example 4 refers to the emulsion fluidcontaining an emulsifer, the table of values for Example 5 describes theproduct used without the addition of an emulsifier.

Example 4

unaged aged material material plastic viscosity [mPa.s] 10 10 yieldpoint [Pa] 16.3 9.1 gel strength [Pa] 10 sec. 10.1 4.8 10 min. 11.9 8.6

Example 5

unaged aged material material plastic viscosity [mPa.s] 9 8 yield point[Pa] 16.8 17.7 gel strength [Pa] 10 sec. 16.8 17.2 10 min. 28.7 14.8

What is claimed is:
 1. A water-based, oil-in-water emulsion drillingfluid suitable for the development of geological formations comprisingan emulsifier, a continuous aqueous phase containing from about 5 toabout 50% by weight of an oil phase dispersed in said aqueous phase,based on the weight of said oil phase and said aqueous phase, said oilphase consisting essentially of a substantially water-insoluble ether ofa mono or polyfunctional alcohol which is fluid or at least plasticallydeformable at use temperature and has a flash point above 80° C.
 2. Awater-based, oil-in-water emulsion drilling fluid as in claim 1 whereinsaid oil phase is present in an amount of from about 8 to about 40% byweight.
 3. A water-based, oil-in-water emulsion drilling fluid as inclaim 1 further containing a a fluid-loss additive, b a weighting agent,c a viscosifier, and d an alkali reserve component.
 4. A water-based,oil-in-water emulsion drilling fluid as in claim 1 further containing anoleophilic alcohol.
 5. A water based, oil-in-water emulsion drillingfluid as in claim 1 further containing a water-insoluble ester oil of amonocarboxylic acid and mono-or polyfunctional alcohol.
 6. Awater-based, oil-in-water emulsion drilling fluid as in claim 5 whereinsaid ester oil is selected from the group consisting of (a) an ester ofa C₁-C₅ monocarboxylic acid and a mono-or polyfunctional alcohol whereinsaid monofunctional alcohol contains at least 6 carbon atoms and saidpolyfunctional alcohol contains from 2 to 6 carbon atoms, (b) an esterof a C₆-C₁₆ aliphatically-saturated monocarboxylic acid and a mono-orpolyfunctional alcohol wherein said monofunctional alcohol contains atleast 6 carbon atoms and said polyfunctional alcohol contains from 2 to6 carbon atoms, and (c) an ester of a C₁₆-C₂₄ mono-or polyolefinicallyunsaturated monocarboxylic acid and a monofunctional alcohol containingat least 6 carbon atoms.
 7. A water-based, oil-in-water emulsiondrilling fluid as in claim 6 wherein said component (c) is selected from(C1) an ester containing at least about 45% by weight of a di-orpolyolefinically unsaturated C₁₆-C₂₄ monocarboxylic acid, and (C2) anester containing less than about 35% by weight of a di-orpolyolefinically unsaturated C₁₆-C₂₄ monocarboxylic acid and at leastabout 60% by weight of mono-olefinically unsaturated C₁₆-C₂₄monocarboxylic acid.
 8. A water-based, oil-in-water emulsion drillingfluid as in claim 5 containing up to about 10% by weight, based on theweight of said oil phase, of an oleophilic basic amine having limitedsolubility in water, is free from aromatic constituents, and has atleast one long-chain hydrocarbon radical containing 8 to 36 carbonatoms.
 9. A water based, oil-in-water emulsion drilling fluid as inclaim 1 having a pH of from about 7.5 to about
 11. 10. A water-based,oil-in-water emulsion drilling fluid as in claim 1 wherein saidwater-insoluble ether of a mono-or polyfunctional alcohol constitutes atleast about 10% by weight of said oil phase.
 11. A water-based,oil-in-water emulsion drilling fluid as in claim 1 wherein saidwater-insoluble ether of a mono-or polyfunctional alcohol is free fromaromatic constituents.
 12. A water-based, oil-in-water emulsion drillingfluid as in claim 1 wherein said monofunctional alcohol contains atleast 4 carbon atoms and up to about 36 carbon atoms.
 13. A water-based,oil-in-water emulsion drilling fluid as in claim 1 wherein said oilphase contains a substantially water-insoluble polyalkylene glycolether.
 14. A water based, oil-in-water emulsion drilling fluid as inclaim 13 wherein said glycol ether is derived from a polyol selectedfrom the group consisting of a lower alkyl diol, ethylene glycol,propylene glycol, and a branched-chain diol having a higher oleophilichydrocarbon radical in the molecule.
 15. A water-based, oil-in-wateremulsion drilling fluid as in claim 1 wherein said oil phase has a pourpoint and setting point below about 0° C. and a flash point of at leastabout 100° C.
 16. A water-based, oil-in-water emulsion drilling fluid asin claim 1 wherein said oil phase has a Brookfield (RVT) viscosity of upto about 2 million mPas at about 20 C.
 17. The process of developing asource of petroleum, natural gas or water by drilling, comprisingdrilling said source in the presence of a water-based, oil-in-wateremulsion drilling fluid comprising an emulsifier, a continuous aqueousphase containing from about 5 to about 50% by weight of an oil phasedispersed in said aqueous phase, based on the weight of said oil phaseand said aqueous phase, said oil phase consisting essentially of asubstantially water-insoluble ether of a mono- or polyfunctional alcoholwhich is fluid or at least plastically deformable at use temperaturesand has a flash point above about 80° C.
 18. The process as in claim 17wherein said oil phase is present in an amount of from about 8 to about40% by weight.
 19. The process as in claim 17 wherein said drillingfluid further contains a a fluid-loss additive, b a weighting agent, c aviscosifier, and d an alkali reserve component.
 20. The process as inclaim 17 wherein said drilling fluid further contains an oleophilicalcohol.
 21. The process as in claim 17 wherein said drilling fluidfurther contains a water-insoluble ester oil of a monocarboxylic acidand a mono-or polyfunctional alcohol.
 22. The process as in claim 21wherein said ester oil is selected from the group consisting of (a) anester of a C₁-C₅ monocarboxylic acid and a mono-or polyfunctionalalcohol wherein said monofunctional alcohol contains at least 6 carbonatoms and said polyfunctional alcohol contains from 2 to 6 carbon atoms,(b) an ester of a C₆-C₁₆ aliphatically-saturated monocarboxylic acid anda mono-or polyfunctional alcohol wherein said monofunctional alcoholcontains at least 6 carbon atoms and said polyfunctional alcoholcontains from 2 to 6 carbon atoms, and (c) an ester of a C₁₆-C₂₄ mono-orpolyolefinically unsaturated monocarboxylic acid and a monofunctionalalcohol containing at least 6 carbon atoms.
 23. The process as in claim22 wherein said component (c) is selected from (C1) an ester containingat least about 45% by weight of a di-or polyolefinically unsaturatedC₁₆-C₂₄ monocarboxylic acid, and (C2) an ester containing less thanabout 35% by weight of a di-or polyolefinically unsaturated C₁₆-C₂₄monocarboxylic acid, and at least about 60% by weight ofmono-olefinically unsaturated C₁₆-C₂₄ monocarboxylic acid.
 24. Theprocess as in claim 21 wherein said drilling fluid contains up to about10% by weight, based on the weight of said oil phase, of an oleophilicbasic amine having limited solubility in water, is free from aromaticconstituents, and has at least one long-chain hydrocarbon radicalcontaining 8 to 36 carbon atoms.
 25. The process as in claim 17 whereinsaid drilling fluid has a pH of from about 7.5 to about
 11. 26. Theprocess as in claim 17 wherein said water-insoluble ether of a mono-orpolyfunctional alcohol constitutes at least about 10% by weight of saidoil phase.
 27. The process as in claim 17 wherein said water-insolubleether of a mono-or polyfunctional alcohol is free from aromaticconstituents.
 28. The process as in claim 17 wherein said monofunctionalalcohol contains at least 4 carbon atoms and up to about 36 carbonatoms.
 29. The process as in claim 17 wherein said oil phase contains asubstantially water-insoluble polyalkylene glycol ether.
 30. The processas in claim 29 wherein said glycol ether is derived from a polyolselected from the group consisting of a lower alkyl diol, ethyleneglycol, propylene glycol, and a branched-chain diol having a higheroleophilic hydrocarbon radical in the molecule.
 31. The process as claim17 wherein said oil phase has a pour point and setting point below about0° C. and a flash point of at least about 100° C.
 32. The process as inclaim 17 wherein said oil phase has a Brookfield (RVT) visocisity of upto about 2 million mPas at about 20° C.